Circuit capable of automatically calibrating a resonance frequency of an antenna and method thereof

Abstract

A circuit capable of automatically calibrating a resonant frequency of an antenna includes the antenna, a switch, a conversion unit, a count comparator, and a capacitor array. The switch is coupled to the antenna for being turned on and turned off according to a pulse. The antenna is used for generating the resonant frequency according to on and off of the switch. The conversion unit is coupled to the antenna for generating a clock according to a signal with the resonant frequency. The count comparator is coupled to the conversion unit for counting a number generated by a reference clock during a period of the clock, and comparing the pulse number with a predetermined value to generate an adjustment signal. The capacitor array is used for adjusting capacitance of the capacitor array according to the adjustment signal.

What is claimed is: 
     
       1. A circuit capable of automatically calibrating a resonant frequency of an antenna, the circuit comprising:
 an antenna having a first terminal, and a second terminal for receiving an electric wave transmitted by a reader; 
 a switch having a first terminal coupled to the second terminal of the antenna, a second terminal coupled to ground, and a third terminal for receiving a pulse; 
 a conversion unit having a first terminal coupled to the second terminal of the antenna for receiving a signal with the resonant frequency of the antenna, and a second terminal for outputting a clock, wherein the conversion unit is used for generating the clock according to the signal, and the clock is a digital square wave; 
 a count comparator having a first terminal coupled to the second terminal of the conversion unit, and a second terminal for outputting an adjustment signal, wherein the count comparator is used for counting a number generated by a reference clock during a period of the clock, and comparing the number with a predetermined value to generate the adjustment signal; 
 a capacitor array for adjusting a value of the capacitor array according to the adjustment signal. 
 
     
     
       2. The circuit of  claim 1 , wherein the antenna comprises:
 an inductor having a first terminal coupled to the first terminal of the antenna, and a second terminal coupled to the second terminal of the antenna; and 
 an antenna capacitor having a first terminal coupled to the first terminal of the antenna, and a second terminal coupled to the second terminal of the antenna, wherein the antenna capacitor is used for shunting the capacitor array to adjust the resonant frequency of the antenna. 
 
     
     
       3. The circuit of  claim 1 , wherein the capacitor array comprises:
 a plurality of capacitor units, each capacitor unit comprising: 
 a capacitor having a first terminal coupled to the first terminal of the antenna, and a second terminal; and 
 a switch having a first terminal coupled to the second terminal of the capacitor, a second terminal coupled to the second terminal of the antenna, and a third terminal coupled to a third terminal of the capacitor array for receiving the adjustment signal, wherein the switch is used for being turned-on and turned-off according to the adjustment signal. 
 
     
     
       4. The circuit of  claim 3 , wherein values of the plurality of capacitors included by the capacitor array are the same. 
     
     
       5. The circuit of  claim 3 , wherein values of the plurality of capacitors included by the capacitor array are different. 
     
     
       6. A method capable of automatically calibrating a resonant frequency of an antenna, the method comprising:
 a switch being turned on and turned off according to a pulse; 
 an antenna generating a resonant frequency according to turning-on and turning-off of the switch; 
 a conversion unit generating a clock according to a signal with the resonant frequency, wherein the clock is a digital square wave; 
 a count comparator counting a number generated by a reference clock during a period of the clock, and comparing the number with a predetermined value to generate the adjustment signal; and 
 a capacitor array adjusting a value of the capacitor array according to the adjustment signal. 
 
     
     
       7. The method of  claim 6 , wherein the capacitor array adjusting the value of the capacitor array according to the adjustment signal comprises:
 the capacitor array turning on and turning off a plurality of switches included by the capacitor array according to the adjustment signal; and 
 adjusting the value of the capacitor array according to turning-on and turning-off of the plurality of switches included by the capacitor array.